This invention relates to zoom systems, and more particularly to a zoom objective lens having a plurality of groups of lens elements movable for zooming, with provision for focusing at one of the groups.
Commonly available zoom objective lenses generally include a front lens, a variator, a compensator and a relay and permit axial movement of the front lens for focusing purposes. As an object to be photographed close up, however, the front lens is usually moved forward to limit the angular field in the object space. For assurance of full aperture of a given diaphragm at all focusing stations, the diameter of the front lens must be increased to a reasonable extent. Compared with the variator and those that follow, therefore, the focusing lens becomes so heavy and bulky that the complete objective turns out not to be suited for use with small photographic cameras.
In order to minimize the size and weight of the objective, focusing may be performed by an independent axial movement not of the front lens but of the compensator, as disclosed in U.S. Pat. No. 4,043,642 assigned to the assignee of the present invention. Such complex movement of the compensator with focusing and zooming may be understood by referring to FIGS. 1 and 2 in which a zoom objective having four lens groups is shown in two different operating positions. The first and fourth groups 11 and 14 are stationary, while the second and third groups 12 and 13 are movable independently of each other. The axial movement of the second group 12 contributes mainly to variation of the focal length of the system, while accommodation for change of object position and the image shift compensation have to be achieved by the axial movement of the third group so as to maintain the system's focal plane stationary in coincidence with the film plane 15 throughout the focusing range and throughout the zooming range.
With the zoom objective set at the minimum focal length and focused for infinity, the lens groups 12 and 13 are assumed to occupy respective reference positions. When the lens group 13 is displaced from this reference position by a distance designated X1 as shown in FIG. 1, the entire system can be focused for an object 10 at a finite distance. In FIG. 2, the second lens group 12 may be displaced a distance designated X2 to vary focal length of the entire system with a simultaneous corresponding image shift from the film plane along the optical axis. The necessary image shift compensation can be effected by the additional axial displacement of the third lens group 13. The total amount of the successive displacements of the third lens group 13 designated by Z in FIG. 2 is varied as a function of not only X2 but also the object distance, S1.
Since the object distance S1 can be estimated by the distance X1, through which the third lens group 13 is moved from the reference position for focusing, the above mentioned function can be defined in the form of Z=F[X2, S1(X1)] with the various parameters determined based on the refractive powers of the individual lens elements and the principal point intervals. Provision is made for automatic adjustment of the third lens group 13 in position during zooming as by use of a microcomputer responsive to electrical detection of the variables X1 and X2 for producing an output signal which is utilized to drive a zoom control mechanism including an electric motor. Thus, the later relative zooming movement of the second and third lens groups 12 and 13 provides a desired range of variation of the focal length of the entire system at any previously adjusted focusing station.
On the other hand, the zoom lens design process may be outlined as comprising the steps of: (1) calling for suitable selection from the available efficient and flexible set of zooming methods such as that which is characterized by linear axial movement of the variator accompanied with reciprocating movement of the compensator during zooming; (2) defining a specific power distribution of the various lens elements in the Gauss region so as to fulfill the requirements concerning with the zoom ratio and physical dimensions; and (3) solving the aberrational problem by computing Seidel's coefficients at three points selected to coincide with the maximum, medium and minimum focal lengths of the entire system and simultaneously by tracing the rays which contribute to production of aberrations, such computation and ray tracing procedure being repeated until good stabilization of aberration correction is obtained throughout the zooming range.
The selection of an appropriate zooming method is a most fundamental item in constructing a zoom objective which is capable of the desired optical performance. However, the present inventors have found that the techniques characteristic of U.S. Pat. No. 4,043,642 are not always efficient and flexible in application to all the types of zoom lenses. In some of the zooming methods, the zoom ratio decreases as the object distance is shortened.